the japanese trapdoors DO NOT lay eggs they give live birth to baby snails the size of a pea, they need a male and female snail to breed so highly unlikely youll polute your local ecosystem with these snails unless you purposely do so. They can survive in an iced over pond because if it gets to cold they hybernate by closing their trap door, and become "rocks" until warmer water.
I haven't had experience with perch and these snais but, at adult size these snails are 2 inches long and an 1 inche wide with a pretty tough spiraled shell and trapdoor they can shut when disturbed.
And all these snails do is graize on my algae in my pond when the water is warm so from spring to fall, when algae grows. they keep the algae down to a extremely low cut f it is the tougher fiber stuff, if it i the algae that wipes off they remove it completely. They are ignored by my pond fish in my signature.
Algae will remain in your pond with these snails but it will be tolerable. the snails do get algae growing on there shells wich helps them blend in to your algae in your pond.
If yah totally want to get rid of algae use algacide, but in my experience this harmed my fish, wiped out some koi and comets with the ****.
I wouldn't eat those clams either.
channel cats are awesome mine does underwater summer salts in the warmer months under the waterfall.
Heres a pick of a japanese trapdoor snail
big huh. thats there max size. can also see their trapdoor. *not me google image.
Also they can be exspensive they are slow breeders and raccons will ignore your fish and go for these snails instead.